Question

The "Ray" model of light fails to explain the optical phenomena when

• A. the light is travelling in any media other than vacuum
• B. the wavelength of the light is very small as compared to the dimensions of openings it interacts with
• C. the wavelength of the light is comparable to the dimensions of the openings it encounters
• D. none of the above.

Answer 1

The correct option is C the wavelength of the light is comparable to the dimensions of the openings it encounters

The validity of ray optics is limited to the situations where 'The wavelength of the light used is very small as compared to the dimensions of the openings it interacts with'. Any phenomena where the wavelength of the light is comparable to the dimensions of the openings, the ray optics model is not able to describe those.
Let's take an example:
Here, in the first case we can use Ray Optics to predict the region of illumination and the observations fit correct as well. It’s a simple way to understand the situation.

In the cases, where wavelength is comparable to the size of obstacles, we don't get the region of illumination the way Ray Optics would have predicted.
Something else happens!!

We get a pattern of illumination and darkness as shown in the diagram.
Yes. Take it as a fact, Ray optics model fails here. So let's learn ray optics and we will unlearn it again to explain these situations.